1. Field of the Invention
The present invention relates to rockets and other self-propelled missiles and projectiles, and more specifically, to solid propellant rocket motors for such devices which are capable of providing multiple propulsive impulses to the vehicle in which they are installed.
2. Description of the Related Art
Motors for rockets, other types of propelled missiles and the like generally are of two types. The first, the liquid propellant motor, has a tank containing a liquid fuel such as liquid hydrogen or ammonia, and a tank containing a liquid oxidizer such as liquid oxygen, nitric acid or fluorine. The two liquids are mixed together in a combustion chamber in a specific proportion and at flow rates designed to cause the liquid to spontaneously combust. The combustion products are expelled from the rocket's exhaust nozzle, thereby providing a thrusting force to propel the rocket. Liquid propellant motors are useful for their ability to be precisely controlled; stopped and restarted; and checked out, fired and calibrated before actual use. Liquid propellant motors also are advantageous in that they provide a wide range of specific impulse ratings, i.e., the amount of thrust per unit mass of fuel burned per unit time; and a relatively long burn time.
The other major type of rocket motor is the solid propellant motor. In solid propellant systems, the rocket is propelled by a solid fuel charge or "grain" that initially is ignited by an electric or pyrotechnic igniting device. As the grain burns, it generates exhaust gases and other combustion products which are expelled through a nozzle at the end of the rocket. The combustion products are expelled from the rocket's exhaust nozzle, thereby providing a thrusting force to propel the rocket. The advantages of the solid propellant motor are its relatively simple structural design and its ease of use.
In many applications, it is desirable to use a solid propellant motor that can provide two separate and distinct propulsive impulses, i.e., a dual pulse motor. For example, the first pulse in a dual pulse motor could be used to fire a missile towards its target. When the missile is near the target, the second pulse could be fired to accelerate the missile, increase its force on impact with the target, and enhance the damage imparted to the target.
Several types of dual pulse motors have been developed. For example, U.S. Pat. No. 4,936,092 to Andrew discloses a system in which grains for the different pulses may be contained in separate, detachable stages. When the grain for one pulse has been entirely consumed, its stage may be jettisoned and a new stage ignited. This arrangement, however, entails the duplication of relatively complicated mechanical parts, the coordination of operations therebetween, and additional weight and manufacturing considerations.
Also, as shown in U.S. Pat. No. 3,122,884 to Grover et al., the grains may be contained in separate combustion chambers within a single stage, where the chambers have separate nozzles or share a common nozzle. Since this scheme still entails some duplication of parts, the propellant load that can be accommodated in the motor is necessarily limited and the cost and weight of the motor are increased. Also, the multiple nozzle configuration limits the size of each nozzle, thereby decreasing the available specific impulse available from the motor.
An alternative to these designs is shown in U.S. Pat. Nos. 3,908,364 to LeFebvre et al. and 4,085,584 to Jones et al. In these systems, the grains for each pulse may be accommodated in a single combustion chamber. To prevent the second grain from igniting once combustion has begun in the first grain, the grains are separated by a thin thermal insulation membrane at their interface. This membrane protects the second grain from inadvertent ignition while the first grain is burning. Once the first grain is spent, a separate igniter initiates combustion of the second grain to begin the second pulse, thereby destroying the membrane and permitting combustion products to exit from the nozzle.
While the above-described prior art systems serve their purpose, they require specialized ignition systems to accommodate the unique combustion characteristics of dual pulse motors. Also, the specialized ignition characteristics of dual pulse motors are fraught with unique problems such as tolerance stack-up and the difficulty of conducting reliable inspections and assemblies.